A lot will depend on what breeds of chickens you have. I have large and bantam breeds so I have a couple sizes.

I have some pics to perhaps help a bit.

This box is preferred by a group of 5 hens. They hated the other design so would lay on the floor of the coop. They use this one quite well. It is 20 inches wide, 16 inches high and 12 inches deep. The board on the front is to keep shavings in and is a 1x6.
This box design would be very simple for pretty much anyone with a saw that can cut a straight line to build. If you click the pic it will be clear that it is plywood with a 1x2 frame around each piece to give a place to put the screws. Pretty easy and very sturdy.

This design is much more complicated and I do not have detailed pics of the insides of it. The slanted roof is to keep them from trying to roost on it.
It uses dishpans as the nest boxes which are very easy to clean or replace. The size of the actual boxes was determined by the dish pans.

You will notice 2 things in common I am sure. They are on the floor. I did that on purpose so that no one decides to lay UNDER the boxes and they are both INSIDE the coop not external. I did that to keep mites from finding a broody hen and make sure moisture stays OUT of them.

The only time any of my chickens ever got mites was in the external nest boxes. I had 5 coops at the time and the 2 that had external boxes were the only ones to have the mites.

A word of caution on the dish pans. Hens will step on the edges of them causing them to tip. This can trap a hen in behind the dish pan and cause it to die. Be very sure they cannot tip the pan by putting anti-tip boards just under the rim of the dish pans or any other light weight tray you end up using.
You can see the anti-tip boards in the pic above. They are simple and life saving.

I hope you get to be home a lot as they are unlikely to like being cooped up. I know you are likely worried about predators and totally understand that. We have a lot of hawks here so mine have a run with a chicken wire roof on it. Just want you to be prepared. OH and chickens that don't want to go back in are the devil to catch lol.

Lots of folks make chicken tractors so that they are not investing hundreds on tall wire and the chickens can be out more. Just a thought.